1. Field of the invention
The present invention concerns mobile telephones. It consists in a broadband multicarrier modulator that can be used in a base transceiver station or in a mobile telephone. In the latter case, the technique developed for the base transceiver station is used with advantage even though the modulator is not normally a multicarrier modulator. The invention enables a relatively large number of mobile telephones to be connected to the same base transceiver station. Generally speaking, the invention is beneficial if a plurality of modulator circuits must be able to modulate a plurality of carriers simultaneously. The invention also consists in a method of programming the multicarrier modulator.
2. Description of the prior art
The invention will be described in the context of a GSM type application but is not limited to that mode of use. Other protocols are equally feasible. The GSM has been chosen because it is sufficiently comprehensive and representative of the problems solved by the invention. FIGS. 1 to 5 show particular features of use of a base transceiver station in a GSM network. The operating constraints of the mobile telephones themselves will be deduced therefrom.
FIG. 1 shows geographical domains. The domains correspond to a district within a town, to a number of streets, for example. The contour of the domains is not necessarily as precise as the hexagons shown. In reality the contour of the domain corresponds to a physical limit beyond which signals transmitted at given frequencies from base transceiver stations within the domain are no longer received. Accordingly, a base transceiver station BTS in a first domain transmits signals on carriers F1 to F8. Other base transceiver stations in adjoining domains transmit on carriers F9 to F16, F17 to F24, F25 to F32, and so on. A further domain adjacent the domains adjacent the first domain could also use the frequencies F1 through F8 of the first domain to broadcast messages to mobile telephones therein. With this system there is little risk of signals getting mixed up.
FIG. 2 shows for one frequency F1 how a time frame (of 4.615 milliseconds duration) is used by eight users. The frame is divided into eight time windows. The users use the frequency F1 to transmit their signals in turn. The time windows have a duration limited to 577 microseconds. Another frequency F2 is used to broadcast messages relating to eight other users, and so on up to frequency F8.
Thus in practice 64 (8.times.8) users can be connected at the same time to the same base transceiver station in one domain. The time distribution of the windows within the frame is known as time-division multiple access (TDMA).
The carrier frequencies are switched at the end of each 4.615 millisecond frame to avoid problems associated with poor quality of propagation of the signal transmitted by a base transceiver station to a mobile on a given carrier (for example carrier F1). Users who were previously using frequency F1 can use another frequency F2. By circular permutation those previously using frequency F2 use frequency F3, and so on.
Eight frequencies from 64 have been retained for a domain in order to solve interference problems that could nevertheless arise between two non-adjoining domains, like those linked by the arrow in FIG. 1. Eight other frequencies from the 64 frequencies are used for the next frame. Those eight other frequencies are necessarily the same as the frequencies of the preceding frame. The principle is substantially the same but immunity to mixing of signals is increased.
FIG. 3 shows the amplitude of the signals transmitted in each of the bands and in each time window, depending on their use. It shows a modulation signal level for a user U1. Nothing is being transmitted for a user U2 who is silent. Another user U3 is far away from the base transceiver station. During an initialization window the base transceiver station recognizes that it is receiving from the user U3 at a low level. It then transmits at a higher level toward that user so that the user can receive their messages directly. The user then transmits a signal whose level corresponds to the effects of the distance from the base transceiver station. With a dynamic range of 30 dB, steps of 1 or 2 dB are adopted in practice, with the result that the level is quantified in terms of 15 or 30 values.
Moreover, although there is no signal being sent to user U2, there is a signal being sent to user U3. However, it is not feasible to increase the transmit level too sharply on passing from the time window of user U2 to that of user U3. In spectral terms, too sharp an increase will be interpreted as the presence of high-level interference frequencies. As shown here, the signal for user U3 is boosted using a substantially bell-shaped envelope to reduce the distortion that occurs in this situation. This neutralizes transmit periods at the beginning and end of the window. This is not a problem in practice because these periods are used for synchronization bits, in any case for bits with no message content.
FIG. 4 shows another constraint on the use of the modulators in the base transceiver stations. Frequencies F1, F4, F14, etc are used in a first frame t1. The bandwidth at each frequency is in practice 200 kHz. Under the GSM standard, what happens in one band must not have effects above a given level in an adjacent band (at a nearby frequency). In practice the standard requires that the signal transmitted in band F1 be attenuated at least 60 dB above 600 kHz, and therefore here in band F4. The bottom of FIG. 4 shows the spectrum of a filter required for a given frequency plan corresponding to a choice of eight frequencies from the 64 frequencies at the time of a given frame t1.
The main problem addressed by the invention is broadband multicarrier transmission. In particular, the invention seeks to generate a multiplicity of carriers which are modulated (F1, F4, F14) with high spectral purity in a given frequency band to feed a final transmit modulator and an amplifier. The problem is obviously particularly severe if the traffic level in the domain is high: if 64 mobile telephones are actually making calls in the domain at a given time.
In the base transceiver stations used at present a narrowband filtering technique is used to produce independent signals on each carrier. Each signal is therefore modulated, filtered and transposed in the analog domain before individual amplification and combination prior to transmission. In a few cases weak signals are combined before overall broadband power amplification.
The problem to be solved is that of finding a method and an architecture for a low cost multicarrier modulator receiving at its input various modulation signals (speech signals) and producing at its output an analytical signal representing the sum of the modulated carriers in the baseband or at an intermediate frequency. Each carrier must be present in the aforementioned sum with the spectral purity required by the constraints of the transmit interface. A posterior analog filtering of each channel is not possible in a broadband radio transmission system. To be satisfactory in applications where the same frequency is re-used the multicarrier modulator must also be able to receive a plurality of modulation signals to be superposed on the same carrier and to control the phase and amplitude of the carrier accordingly.
The usual problem encountered in digital methods is that of the number of operations effected, especially multiply and accumulate (MAC) operations on analytical numbers (complex numbers). The problem encountered with one carrier is obviously intensified if there is more than one carrier. The invention seeks to process simultaneously 64 carriers in a 25 MHz band with a sampling rate around 100 MHz, for example.
A prior art method of reducing digital MAC calculations transforms some of the operations into tabulations. In this case tables must be synthesized on the basis of modulation and filtering constraints. It is not obvious how to do this manually or even using conventional computer means, because of the size of the tables and the encoding load that this implies. What is more, the skilled person is still a long way from being able to do this independently of the modulation scheme and filtering constraints.
FIG. 5 summarizes current best practice in the field of multicarrier modulators. A signal that has previously undergone compression, redundancy, channel or other encoding is available in digital form. It is spectrally synthesized. An encoder receives the bits of the signal and, depending on preceding bits of the same signal (to simplify the example two previous bit times are considered here), produces encoded signals, symbols, resulting from linearizing the modulation over a time period equal to the duration of three bits. In an example to the GSM standard, the signal bits are delivered at a frequency of 13/48 MHz (substantially equal to 270 kHz). The encoder delivers the symbols at the same rate. A conversion table then delivers instantaneous frequency samples corresponding to the symbols produced by the encoder.
A phase accumulator connected to the conversion table therefore receives a signal proportional to the frequency. The accumulator adds a sample received to its content and returns the result to its input. The accumulator therefore produces an instantaneous phase. It is preferable to oversample the output of the phase accumulator by operating it at a significantly higher speed, for example in the range from 8 to 32 times its speed, rather than to have at the output of the phase accumulator only an instantaneous phase at the same rate as that at which the bits of the signal are produced. This means that the table converting symbols into frequency values and the phase accumulator are operated at 16.times.13/48=13/3 MHz, for example. The instantaneous frequency value is normalized to correspond to a phase difference that will occur during an oversampling period.
With this benefit of an instantaneous phase, signals corresponding to the instantaneous amplitudes of a modulating signal can be produced using sine and cosine transformation tables. All that then remains is to transform these instantaneous amplitude signals by means of digital-analog converters (DAC) and to feed them into the mixers to produce the signal to be transmitted. The mixers receive the signal from a local oscillator in phase quadrature. The signals produced by the mixers are then mixed in a third mixer which is also an amplifier. The amplifier is connected to an antenna to transmit the signal. The analog filtering previously considered is shown in FIG. 5 by a (narrowband) bandpass filter represented in dashed outline. Clearly a filter of the above kind is not feasible in a multicarrier modulator, especially if the spectral plan changes from one frame t1 to the other t2.
A frequency jump must be provoked using an adder to obtain a carrier Fi programmable from a signal at a frequency around zero. Intercollating the adder between the conversion table and the phase accumulator in order to superpose on each sample period the frequency of the carrier programmed at the instantaneous modulation frequency is known per se. This is shown with the adder, hopper, requiring the injection of a signal at a frequency F1 to F64 between the converter circuit and the phase accumulator.
Of course, to effect this addition without aliasing the oversampling rate must be greater than the maximum synthesized frequency. Here the maximum synthesized frequency is 26 MHz. This maximum frequency corresponds to 64 bands each of 200 kHz (i.e. 12.8 MHz) multiplied by 2 to satisfy the Nyquist criterion. An oversampling rate of 4 is chosen in this case and yields an oversampling frequency of 104 MHz. There is no simple way to provide a channel filter after synthesis because the frequency can change from one frame to another (see FIG. 2). The analog filter must be replaced by a digital filter to eliminate interference generated by modulation outside the wanted 200 kHz band.
With IIR type filters with 20 coefficients, phase distortion and computation dynamic range problems lead to 2080 million (104 MHz.times.20) complex multiplications and accumulations (MCMAC) per second. There is no phase distortion problem with FIR type filters but the computing capacity required is 54080 million real multiplications and accumulations (MRMAC). One CMAC operation is equivalent to four RMAC operations.
FIG. 5 uses dashed line frames to show that to combine a plurality of carriers simultaneously it is necessary to add signals produced by a plurality of modulator circuits before converting the result of the combination in a digital-analog converter and before modulating the signal from a local oscillator by the added signals in the phase quadrature mixers. Obviously eight modulator circuits would be required for the eight carriers, all oversampled at 104 MHz. There would then be eight filter operations, which would lead in this latter case to 432640 MRMAC. This type of filtering is virtually impossible at present because it would be too costly and would consume much too much power. In the current state of the art, digital signal processors are capable of only 400 MMAC per unit and consume one watt. This means that the power consumption for the eight channels would be 1 kW.
This is not at all competitive with an analog solution, even if particularly judicious integration were able to produce a saving by a factor of 4 to 10.
Also, implementing discrete computing elements would lead to a loss of efficiency, to a loss of power due to the many high-speed input-output interfaces that would be required between the various computing units. Clearly it is not feasible to step up to 32 or 64 carriers, as envisaged by the invention.
One aim of the invention is therefore to remedy the above drawbacks by limiting the cost and therefore the quantity of equipment used and the power consumed by the various circuits employed. Another aim of the invention is to render the multicarrier modulator integratable and modular by incorporating channel filters into the modulator.
The principle on which the invention is based is that of comprehensive tabulation of all possible complex trajectories of filtered signals that can be deduced from combinations of bits or symbols fed into the modulator.
The invention is based on the following observations: linear modulation has a finite number of states and transitions (possible signal trajectories during a period of one bit: symbol period). An FIR filter is equivalent to a finite state machine (also known as an automation or sequential system) if its response time is finite. Finally, a combination, cascade or convolution of two finite state machines remains a finite state machine but its complexity is the product of the initial two complexities.
Because the input modulation also has a finite number of states and transitions (for bit level modulation there are two possible states and therefore two possible transitions), the output of a GMSK (Gaussian Minimum Shift Keying) type linear modulator filtered a posterior by an FIR type filter has a finite number of states and trajectories corresponding to a bit period.
Accordingly, a state machine (or automation) can be constructed. A generic automation of the above kind is based on a table whose read address is partly based on the information read in a previous read operation. Also, there is only one trajectory signal for each new state or symbol.
Accordingly, even if the table is very large, there is a completely tabulated solution, i.e. a solution requiring no MAC type operations. As will emerge below, the modulator can be entirely implemented with two tables in cascade, one for producing future states at the end of the bit/symbol period and another, pre-computed, table called the trajectory table which produces directly the instantaneous signals to be transmitted. The trajectory table feeds a digital-analog converter connected to the mixers directly. It supplies fully processed signals, i.e. signals incorporating modulation, frequency hops and filtering. The tables of the automaton of the invention are significantly smaller than the tables previously referred to precisely because of the presence of the automaton.
Another feature of the invention is that the automaton produces arbitrary encoded states from the bit of the signal to be transmitted in order to simplify production of the pre-computed table. These arbitrary encoded states are then allocated to instantaneous signal values to be transmitted obtained by computation from the bit of the signal to be transmitted.